The following, written by Rabbi Moses ben Jacob of Coucy, author of the Sefer Mitzvos Gadol on positive Mitzvah # 74.
And I have already expounded to Galus Yerushalayim in Spain (Sefard) and the other Galuyos under Christianity (Edom), that now that the Galus has lasted too long a Jew must separate himself from the frivolities of (Hevlei) the world and grasp the seal of HaKadosh Baruch Hu, which is Truth, and not lie, neither to Jews nor to non-Jews, nor to deceive them in any matter, and to sanctify ourselves even in that which is permissible to us, as it says, (Tzefania 3,13): “The remnant of Israel will not commit foul deeds nor speak falsehoods, nor will there be found in their mouths treacherous tongues.” And then, when HaKadosh Baruch Hu comes to redeem us, the non-Jews will say that He is just in doing so, for we are men of truth and Toras Emes is in our mouths.
But if we conduct ourselves towards the non-Jews with deceit (Ramaus), then they will say: “See what HaKadosh Baruch Hu has done, that He has chosen as His portion thieves and cheats.” Furthermore, it is written (Hoshea 2,25): “And I will plant them in the earth.” Why does a person plant a measure of grain in the earth? A person plants in order to cultivate several measures. So too, HaKadosh Baruch Hu planted Yisroel in the various lands so that converts would join us. As long as we conduct ourselves amongst them with deceit who will cling to us? And, we find that HaKadosh Baruch Hu was upset even by theft from evildoers, as it says (Bereishis 6,11): “And the land was filled with theft (Chamas).”
Further, I bring proof from the Yerushalmi Chapter Eilu Metzios (Halacha 5), where it says: “The elder rabbis (Rabbanan Saviai) bought a measure of grain from non-Jews and found within it a bundle of money. They returned it to them, and the non-Jews said: ‘Blessed is the G-d of the Jews.’” Many similar stories of lost items that were returned to non-Jews because of Kiddush HaShem are related there.
